Canon New 70-300L vs Old 100-400L

OK I know I'm not asking for a like for like comparison and there is precious little testing done of the new Canon 70-300L however...

I have a Canon 550D with excellent 15-85 IS USM but want to replace my 70-300 IS USM which is soft at 300, plasticy and recently broke down. Sense says wait and see reviews but I am going to The Gabia hopefully at end of Nov and want a good lens for wildlife. The (real) price of new 70-300 L lens is only marginally higher than old 100-400 and not an issue. Does 100-400 have similar fast ring USM?

I want to avoid use of tripod (not much use on a boat or dugout canoe) How does Canon's 4 stop IS on 70-300L compare to 100-400L?

(Not sure how close you can get to birds in Gambia) Will I find the 100mm loss on 70-300L a real issue?